This invention lies in the area of cardiac pacers and the method of operation of same, with programmed means for varying pacemaker operation and, in particular, pacemaker systems and other implantable systems with microprocessor control for switching between unipolar and bipolar operation in accordance with programmed events such as sensing of patient signals and stimulus delivery.
In the field of cardiac pacemakers, there is a need to provide an efficient electrode system for the operations of delivery of stimulus pulses and sensing patient heartbeat signals. In a conventional single chamber demand pacemaker, of the demand type, the electrode system must provide for efficient delivery of ventricular stimulus signals, and also provide efficient pick up of natural QRS signals. In dual chamber pacemakers, there is additionally a need to be able to sense atrial signals, and to deliver atrial stimulus pulses from the pacemaker. In another type of pacemaker, as set forth in U.S. Pat. No. 4,228,803, assigned to the same assignee, there is a need to sense T-waves.
Generally there are two types of electrode systems which are used in cardiac pacing. In the bipolar system of operation, two electrodes are positioned near the tip of a lead which is placed into the heart, the electrodes typically being rings which conduct the stimulus pulse and sense natural cardiac signals. In a unipolar electrode arrangement, a single electrode is placed on the lead, preferrably out or near the tip, and an indifferent electrode is utilized at a location remote from the electrode tip. Most typically, in unipolar, or monopolar arrangements, the indifferent electrode is a portion of the pacemaker housing, which is a convenient way of obtaining a large surface indifferent electrode. Alternately, the indifferent electrode may be positioned on the lead itself, at a point proximal to the tip, as is known in the art.
The pacemaker industry has not resolved, and indeed cannot resolve the question of whether unipolar or bipolar operation is generally preferrable. Some physicians adhere to one or the other modes of operation, having their own reasons for doing so. Most pacemaker manufacturers have provided pacing systems for operation in each of the modes, such that the physician can choose a unipolar or a bipolar system. Bipolar leads and systems have the advantages of reduced pick up of electromagnetic interference, and they avoid the problem of unipolar systems wherein the pectoral muscle can be inadvertantly excited. Unipolar systems are generally recognized to have the advantage of better sensitivity for sensing heart signals, and to have reduced polarization problems due to the relatively large indifferent electrode and the relatively small lead electrode. In special applications, such as searching for an evoked response signal (QRS wave) following delivery of a stimulus pulse, or in measuring T-waves, the unipolar system is definitely preferable. There could be a problem if, in a bipolar system, the heart muscle is captured at both electrodes, in which case the sensed T-wave and QRS wave would be simply the difference of the two signals and would be very small.
In view of the above, it is seen that there is a need to provide increased flexibility, by which the pacer system can utilize the advantages of each of the bipolar and unipolar modes of operation for the particular events that are taking place. Thus, there is a need for a system which automatically switches between unipolar and bipolar forms of operation, the switching being programmed for optimal operation of the different pacer events which take place during each pacing cycle. The desirability for switching applies to both single chamber and dual chamber pacing systems. In dual chamber systems, it may be desirable, for example, to have bipolar operation in the atrium to reduce the need for blanking following delivery of the ventricular stimulus, while at the same time having a unipolar electrode arrangement for sensing P waves. Generally, considerations for unipolar and bipolar sensing vary at different times in the pacing cycle, dependent upon the next anticipated event.
While this invention is described in terms of switching between unipolar and bipolar operation in a pacemaker system, it is to be understood that it generally applies to optimized switching of various electrode systems, for pacemakers and other types of devices for delivering stimuli to a patient and/or sensing patient signals. For example, electrode configurations may be used which are not termed unipolar or bipolar, as those terms are understood in the pacemaker art, but which electrode configurations are changed during cycles of operation of the system.